Birefringent marking

ABSTRACT

The invention relates to a birefringent marking comprising a liquid crystal material. The invention further relates to method of providing such a birefringent marking and to the use of the birefringent marking for decorative or security applications.

FIELD OF THE INVENTION

[0001] The invention relates to a birefringent marking comprising aliquid crystal material. The invention further relates to method ofproviding such a birefringent marking and to the use of the birefringentmarking for decorative or security applications.

BACKGROUND AND PRIOR ART

[0002] The use of birefringent films as security devices has beenreported in prior art. GB 2328180 discloses a security device comprisinga birefringent film having regions of different thickness, such thatwhen the material is viewed through a polariser, a pattern is exhibited.The birefringent film is e.g. a stretched polymer film like a PET film.The pattern is achieved e.g. by forming recesses in the birefringentfilm, or by bonding the film to a higher melting point layer assupporting layer and reducing the thickness or destroying thebirefringence in selected regions of the birefringent film. Destructionof the birefringence is achieved e.g. by melting, hot stamping, thermalprinting or laser writing. Reduction of the film thickness is achievede.g. by removing parts of the film or by laser writing. The highermelting point supporting layer then holds the partially destructedbirefringent film in place. GB 2328180 further reports that a reflectivelayer can be applied to the birefringent film after provision of thepattern, e.g. by metallisation or printing of a reflective material.

[0003] The device according to GB 2328180 has several drawbacks. Forexample, its manufacture is complicated and require a series ofmanufacturing and processing steps like forming the pattern on thebirefringent film, lamination to a high melting point supporting layerand application of the reflective layer. The methods of forming thepattern are also complicated and require additional technical effort andequipment, like the use of high temperatures or lasers. Furthermore,stretched plastic films used as birefringent layers as described in GB2328180 do generally exhibit only moderate birefringence. Thereforeusually a high film thickness is required to achieve a satisfyingoptical effect. The film thickness is further increased by the use ofthe supporting layer. Thus, GB 2328180 reports devices with a combinedfilm thickness of the birefringent and supporting layer of about 15 μm.Such a high thickness is critical especially for applications in thesecurity sector, like e.g. security markings to be applied on documentsof value or banknotes.

[0004] The aim of the present invention is to provide a birefringentmarking, in particular for decorative and security applications, thatdoes not have the drawbacks of the prior art devices, is easy tomanufacture and can be applied to a broad variety of substrates,surfaces and objects.

[0005] The inventors of the present invention have found that the aboveaims can be fulfilled by providing a birefringent marking comprising aliquid crystal (LC) material having discrete regions with differentthickness. When this birefringent marking is viewed through a polariser,an image is seen that appears to rotate when the polariser is rotated.In particular striking visible effects can be achieved by providing thebirefringent marking onto a reflective layer, like for example a metallayer, and/or by providing an optical phase shift or retardation layerbetween the birefringent liquid crystal layer and the reflective layer.The LC material can easily be provided e.g. directly onto a reflectivesubstrate, thus a security marking can be prepared in an easy way,without the need of complicated methods or techniques and expensiveequipment.

SUMMARY OF THE INVENTION

[0006] One object of the invention is a birefringent marking comprisingat least one layer of liquid crystal material having regions ofdifferent thickness.

[0007] Another object of the invention is a method of preparing abirefringent marking as described above and below by applying dropletsof a solution of a liquid crystal material to a substrate and allowingthe solvent to evaporate.

[0008] Another object of the invention is a method of applying a hiddenimage to a surface or substrate, said image being invisible when viewedunder unpolarised light and visible when viewed under polarised light,by providing a birefringent marking comprising a liquid crystal materialhaving regions of different thickness to the surface.

[0009] Another object of the invention is the use of a birefringentmarking as described above and below in optical elements, coatings,lacquers, inks or paints for cosmetic, decorative or securityapplications.

[0010] Another object of the invention is a security marking or devicecomprising a birefringent marking as described above and below.

[0011] Definition of Terms

[0012] The term ‘film’ as used in this application includesself-supporting, i.e. free-standing, films that show more or lesspronounced mechanical stability and flexibility, as well as coatings orlayers on a supporting substrate or between two substrates.

[0013] The term ‘liquid crystal or mesogenic material’ or ‘liquidcrystal or mesogenic compound’ should denote materials or compoundscomprising one or more rod-shaped, board-shaped or disk-shaped mesogenicgroups, i.e. groups with the ability to induce liquid crystal phasebehaviour. The compounds or materials comprising mesogenic groups do notnecessarily have to exhibit a liquid crystal phase themselves. It isalso possible that they show liquid crystal phase behaviour only inmixtures with other compounds, or when the mesogenic compounds ormaterials, or the mixtures thereof, are polymerised.

DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention relates to a method of incorporating avariable birefringent layer to a reflective section of a document, forexample a banknote, such that a non regular pattern is seen when viewedthrough a linear polariser. The birefringent marking and its method ofpreparation according to the present invention have several advantagesover prior art, such as

[0015] the variation of the birefringent layer thickness can be formedwithout any processing such as laser etching

[0016] the birefringent material can be applied to any non-porousreflective surface without prebonding to other materials,

[0017] the birefringent material can be applied by conventional printingor coating techniques,

[0018] the birefringent material exhibits a multicoloured pattern whenviewed through a linear polariser,

[0019] the coloured pattern appears to rotate when the polariser isrotated.

[0020] Preferred embodiments of the invention relate to a birefringentmarking wherein

[0021] the liquid crystal material is a vitrified, polymerised orcrosslinked liquid crystal material,

[0022] the liquid crystal material is a nematic or smectic liquidcrystal material,

[0023] the liquid crystal material is a cholesteric liquid crystalmaterial, the birefringent marking further comprises a reflective layerbetween the liquid crystal layer and the substrate, and/or the substrateis a reflective substrate,

[0024] the reflective layer or substrate is a metallic or metallizedlayer, hot stamping foil, holographic image, pearlescent or interferencelayer or pearlescent or interference pigment,

[0025] the reflective layer comprises one or more interference pigments,preferably provided in a light transmissive binder,

[0026] the reflective layer in addition to the interference pigmentsadditionally comprises one or more further pigments or dyes,

[0027] the birefringent marking further comprises an absorptive layerbetween the liquid crystal layer and the substrate, and/or the substrateis an absorptive layer,

[0028] the birefringent marking further comprises at least one opticalretardation layer on the side facing away from the substrate,

[0029] said optical retardation layer is a quarter wave retardationlayer,

[0030] said optical retardation layer is a stretched or compressed filmof isotropic polymer,

[0031] the birefringent marking further comprises a linear polariser onthe side facing away from the substrate,

[0032] said linear polariser and/or optical retardation layer comprise avitrifed, polymerised or crosslinked liquid crystal material withuniform orientation.

[0033] According to a preferred method of the present invention thebirefringent marking is prepared by applying droplets of a mixturecomprising a liquid crystal (LC) material and a solvent to a substrateor the surface of an object and allowing the solvent to evaporate.

[0034] The droplets preferably contain a mixture of the LC material andan organic solvent. The LC mixture can be a solution, dispersion oremulsion of the LC material in the solvent. Preferably a solution isused. Evaporation of the solvent from the applied LC droplets createsdifferent thickness in the final LC layer, thus providing a non-uniformfeature which is very difficult to copy. In this way, it is possible toprepare multiple security markings from one sample of LC material by thesame method, wherein each marking has a unique pattern, like afingerprint, which can be easily identified and is difficult to forge.

[0035] The LC mixture can be appplied in fomr of droplets to thesubstrate by known techniques. In some cases it is suitable to heat theLC mixture in order to facilitate the evaporation of the solvent. The LCmixture can also be applied by other methods that are known in the art,like for example ink-jet, offset gravure and screen printing.

[0036] It is also possible to apply the LC mixture onto the substrate bymeans of a stamp.

[0037] The thickness of the LC layer can be altered for example bychanging the solvent concentration of the LC mixture. The thickness ofthe final LC layer thus increases with increasing amount of LC materialin the mixture. Another factor influencing the thickness of the LC layeris the way in which the applied droplets disperse on the substrate,which is related to the surface energy of the substrate and the surfacetension of the LC droplets. These parameters can be changed by varyingthe substrate material or by varying the solvent and/or liquid crystalmaterial. It is possible e.g. to alter the surface energy of thesubstrate by applying a layer of a for example a surface active materialor an alignment material to the substrate. Alternatively e.g. a wettingagent or a surfactant can be added to the liquid crystal solution tochange the surface tension of the LC droplets and their adhesion to thesubstrate. Suitable surfactants are described for example in J. Cognard,Mol. Cryst. Liq. Cryst. 78, Supplement 1, 1-77 (1981). Particularlypreferred are non-ionic surfactants, such as the commercially availablefluorocarbon surfactants Fluorad 171 (from 3M Co.), or Zonyl FSN (fromDuPont).

[0038] The birefringent marking is preferably applied onto a reflectivesurface or substrate. In a preferred embodiment, the birefringentmarking already comprises the reflective layer on the side of the liquidcrystal layer facing away from the viewer. The birefringent marking isthen applicable to any kind of reflective non-reflective surface orsubstrate.

[0039] For the reflective substrate or layer in principle any reflectivematerial can be used. Thus, the reflective layer is e.g. a metal ormetallised layer, hologram, kinegram, hot stamping foil, pearlescent orinterference pigment, or a layer comprising metal, metallised,pearlescent or interference pigments in a transparent binder.

[0040] Metal or metallised films or layers can be selected e.g of Al,Cu, Ni, Ag, Cr or alloys like e.g. Pt-Rh or Ni-Cr, or layers comprisingone or more metal flakes dispersed in a light transmissive binder.Suitable metal flakes are e.g. flakes aluminium, gold or titan, or metaloxide flakes of e.g. Fe₂O₃ and/or TiO₂. Suitable pearlescent orinterference pigments are e.g. mica, SiO₂, Al₂O₃, TiO₂ or glass flakesthat are coated with one or more layers of e.g. titanium dioxide, ironoxide, titanium iron oxide or chrome oxide or combinations thereof,flakes comprising combinations of metal and metal oxide, metal flakes ofe.g. aluminium coated with layers of iron oxide layers and/or siliciumdioxide. It is also possible to use liquid crystal pigments or coatingscomprising a polymerized or crosslinked liquid crystal material, e.g.cholesteric liquid crystal pigments as described in U.S. Pat. No.5,364,557, U.S. Pat. No. 5,834,072, EP 0 601 483, WO 94/22976, WO97/27251, WO 97/27252, WO 97/30136 or WO 99/02340.

[0041] It is also possible to use a reflective substrate or layercomprising a hologram or kinegram, a holographic layer with an embossed,patterned or structured surface, or a layer of reflective holographicpigments. Light reflected by higher regions of the structured surfacewill interfer with light reflected by lower regions of the structuredsurface, thereby forming a holographic image.

[0042] Thus, for example a substrate like e.g. a banknote, or selectedregions thereof, can be printed or coated with a hologram or reflectivemetal layer, which is then covered by a birefringent marking asdescribed above. This embodiment is particularly suitable for use asfalse-proof security threads or holograms on banknotes or documents ofvalue, providing a security marking by which the banknote is easy toauthenticate when viewed through a polariser.

[0043] In an alternative embodiment of the present invention thebirefringent marking is applied to a transparent substrate and viewed intransmission between two polarisers.

[0044] Another preferred embodiment of the present invention relates toa birefringent marking wherein an optical phase shift or retardationlayer is provided between the birefringent LC layer and the reflectivesubstrate. The retardation layer causes an additional phase shift of thelight reflected by the reflective substrate. This leads to an additionalcolour shift of the birefringent marking when viewed through apolariser. Preferably, the retardation layer is a quarter wave film orfoil (QWF) exhibiting a net retardation that is approximately 0.25 timesthe wavelength transmitted by the circular polarising layer.

[0045] As a retardation layer, it is possible to use uniaxially orbiaxially stretched or compressed films of an isotropic polymer, likee.g. polyethylene terephthalate (PET), polyvinyl alcohol (PVA),polycarbonate (PC), di- or triacetyl cellulose (DAC, TAC). For examplePET films are commercially available from ICI Corp. under the trade nameMelinex. Especially preferred are PVA and PET films.

[0046] It is also possible to use a phase shift layer or retardationfilm comprising vitrified, polymerised or crosslinked liquid crystallinematerial with planar orientation, i.e. with the mesogenic groups of theliquid crystal material being oriented substantially parallel to theplane of the layer into a preferred direction. A retardation filmcomprising polymerised LC material with planar orientation is describedin WO 98/04651, the entire disclosure of which is incorporated into thisapplication by way of reference. It is also possible to use an opticalretardation film comprising one or more layers of a polymerised liquidcrystalline material with tilted orientation, i.e. with the mesogenicgroups of the liquid crystal material are oriented at an oblique anglerelative to the plane of the layer into a preferred direction. Such aQWF is described in WO 98/12584, the entire disclosure of which isincorporated into this application by way of reference.

[0047] The retardation layer can also comprise platelet shapedmicroflakes of a light retarding material as mentioned above. Thus, e.g.a retardation film of a stretched polymer or polymerised LC material canbe ground into small flakes which are then incorporated into a lighttransmissive binder system, and coated onto the reflective layer, toform the retardation layer.

[0048] In case the reflective substrate is a holographic layer asdescribed above, the use of an additional phase shift or retardationlayer leads to an to improved colour play and to an improved visibilityof the holographic image, which is otherwise often difficult torecognize especially in a bright environment.

[0049] The birefringent marking according to the present inventioncontains a pattern or image that is invisible when viewed underunpolarised light and becomes visible only when viewed through apolariser. In a preferred embodiment, the birefringent marking alreadycomprises a linear polariser on top of the liquid crystal layer, i.e. onthe side facing away from the reflective substrate. The birefringentmarking is then directly visible to the eye, without the use of aseparate polariser. Such permanently visible birefringent markings aredesired for specific applications.

[0050] For the linear polariser in principle all materials known in theart are suitable. Thus, e.g. standard linear absorption polarisers canbe used comprising an uniaxially stretched polymer film of e.g.polyvinyl alcohol, or comprising a polymer film into which isincorporated a dichroic dye. It is also possible to use a linearpolariser comprising a vitrified, polymerised or crosslinked liquidcrystal (LC) material that exhibits macroscopically uniform planarorientation, i.e. with the mesogenic groups of the LC material beingoriented substantially parallel to the plane of the layer into apreferred direction. The linear polariser can also be prepared e.g. bycoating a layer of polymerisable LC material comprising a dye onto asubstrate, aligning the LC material into planar orientation, i.e. sothat the mesogenic groups are oriented parallel to the plane of thelayer, polymerising or crosslinking the material by exposure to heat oractinic radiation. Linear polarisers made from polymerisable material bythe above method are described in EP 0 397 263 (Philips), the entiredisclosure of which is incorporated into this application by way ofreference.

[0051] The birefringent marking may also comprise a birefringent LClayer with regions of different thickness sandwiched between twopolarisers, and optionally one or more retardation layers located on oneor both sides of the LC layer between the LC layer and the polariser.

[0052] In another preferred embodiment the LC material of thebirefringent marking is a cholesteric LC (CLC) material. Cholesteric LC(CLC) materials with planar orientation show reflection of circularpolarised light. Thus, a birefringent marking comprising a CLC materialwith regions of different thickness prepared according to the inventivemethod will exhibit a hidden image that is visible when viewed through acircular polariser. CLC materials are preferably used with dark or blacksubstrates, however, reflective substrates can also be used. In case theLC layer comprises a CLC material reflecting selected wavelengths oflight, the reflection colour is visible on a dark background without apolariser. In case the CLC layer is a layer reflecting a broadwavelength band, or reflecting the entire visible spectrum, so that nospecific reflection colour is seen, the pattern can be made visible byviewing through a circular polariser. Broad waveband CLC films orcoatings and their preparation are described e.g. in EP 0 606 940, WO97/35219, EP 0 982 605 and WO 99/02340.

[0053] The LC material of the birefringent layer is preferably anematic, smectic or cholesteric LC material. Nematic LC materials areespecially preferred. In case of a cholesteric LC material, preferably asubstrate or surface comprising a light absorbing material, like a darkor black substrate, is used instead of the reflective substrate, and acircular polariser is used instead of a linear polariser.

[0054] The liquid crystal material of the birefringent marking ispreferably a polymerisable or crosslinkable material that is polymerisedor crosslinked during or after evaporation of the solvent.

[0055] Alternatively it is possible to apply droplets of a solution oremulsion of a liquid crystal polymer in a suitable solvent. LC sidechain polymers or LC main chain polymers can be used. LC side chainpolymers are especially preferred. For example, LC side chain polymerscomprising a polyacrylate, polymethacrylate, polysiloxane, polystyreneor epoxide backbone with laterally attached mesogenic side chains can beused. The polymer may also comprise side chains with reactive groupsthat can be crosslinked after or during evaporation of the solvent. Ifpolymers with a glass temperature that is higher than ambienttemperature are used, evaporation of the solvent leaves a solid LCpolymer film. The LC polymer may also be subjected to mechanical or heattreatment after application to the substrate, thereby differences inthickness between the different regions and fomation of the image can becontrolled. In case LC polymers with high melting points are used, likee.g. LC main chain polymers, it is also possible to apply droplets of amelt of the LC polymer onto the substrate which solidify upon cooling.

[0056] In case a polymerisable LC material is used, it preferablycomprises at least one polymerisable mesogenic compound having onepolymerisable functional group and at least one polymerisable mesogeniccompound having two or more polymerisable functional groups.

[0057] In another preferred embodiment the polymerisable LC materialcomprises polymerisable mesogenic compounds having two or morepolymerisable functional groups (di- or multireactive or di-ormultifunctional compounds). Upon polymerisation of such a mixture athree-dimensional polymer network is formed, which is self-supportingand shows a high mechanical and thermal stability and a low temperaturedependence of its physical and optical properties. By varying theconcentration of the multifunctional mesogenic or non mesogeniccompounds the crosslink density of the polymer film and thereby itsphysical and chemical properties such as the glass transitiontemperature, which is also important for the temperature dependence ofthe optical properties of the polymerised film, the thermal andmechanical stability or the solvent resistance can be tuned easily.

[0058] The polymerisable mesogenic mono-, di- or multireactive compoundscan be prepared by methods which are known per se and which aredescribed, for example, in standard works of organic chemistry such as,for example, Houben-Weyl, Methoden der organischen Chemie,Thieme-Verlag, Stuttgart. Typical examples are described for example inWO 93/22397; EP 0 261 712; DE 19504224; DE 4408171 and DE 4405316. Thecompounds disclosed in these documents, however, are to be regardedmerely as examples that do not limit the scope of this invention.

[0059] Examples representing especially useful monoreactivepolymerisable mesogenic compounds are shown in the following list ofcompounds, which should, however, be taken only as illustrative and isin no way intended to restrict, but instead to explain the presentinvention:

[0060] Examples of useful direactive polymerisable mesogenic compoundsare shown in the following list of compounds, which should, however, betaken only as illustrative and is in no way intended to restrict, butinstead to explain the present invention

[0061] In the above formulae, P is a polymerisable group, preferably anacryl, methacryl, vinyl, vinyloxy, propenyl ether, epoxy or stytrylgroup, x and y are each independently 1 to 12, A is 1,4-phenylene thatis optionally mono- di or trisubstituted by L¹ or 1,4-cyclohexylene, vis 0 or 1, Z⁰ is —COO—, —OCO—, —CH₂CH₂— or a single bond, Y is a polargroup, R⁰ is an unpolar alkyl or alkoxy group, and L¹ and L² are eachindependently H, F, Cl, CN or an optionally halogenated alkyl, alkoxy,alkylcarbonyl, alkoxycarbonyl or alkoxycarbonyloxy group with 1 to 7 Catoms.

[0062] The term ‘polar group’ in this connection means a group selectedfrom F, Cl, CN, NO₂, OH, OCH₃, OCN, SCN, an optionally fluorinatedcarbonyl or carboxyl group with up to 4 C atoms or a mono- oligo- orpolyfluorinated alkyl or alkoxy group with 1 to 4 C atoms. The term‘unpolar group’ means an alkyl group with 1 or more, preferably 1 to 12C atoms or an alkoxy group with 2 or more, preferably 2 to 12 C atoms.

[0063] Polymerisation of the polymerisable LC material takes place byexposing it to heat or actinic radiation. Actinic radiation meansirradiation with light, like UV light, IR light or visible light,irradiation with X-rays or gamma rays or irradiation with high energyparticles, such as ions or electrons. Preferably polymerisation iscarried out by UV irradiation. As a source for actinic radiation forexample a single UV lamp or a set of UV lamps can be used. When using ahigh lamp power the curing time can be reduced. Another possible sourcefor actinic radiation is a laser, like e.g. a UV laser, an IR laser or avisible laser.

[0064] The polymerisation is carried out in the presence of an initiatorabsorbing at the wavelength of the actinic radiation. For example, whenpolymerising by means of UV light, a photoinitiator can be used thatdecomposes under UV irradiation to produce free radicals or ions thatstart the polymerisation reaction. When curing polymerisable mesogenswith acrylate or methacrylate groups, preferably a radicalphotoinitiator is used, when curing polymerisable mesogens vinyl andepoxide groups, preferably a cationic photoinitiator is used. It is alsopossible to use a polymerisation Initiator that decomposes when heatedto produce free radicals or ions that start the polymerisation. As aphotoinitiator for radical polymerisation for example the commerciallyavailable Irgacure 651, Irgacure 184, Darocure 1173 or Darocure 4205(all from Ciba Geigy AG) can be used, whereas in case of cationicphotopolymerisation the commercially available UVI 6974 (Union Carbide)can be used. The polymerisable LC material preferably comprises 0.01 to10%, very preferably 0.05 to 5%, in particular 0.1 to 3% of apolymerisation initiator. UV photoinitiators are preferred, inparticular radicalic UV photoinitiators.

[0065] The curing time is dependant, inter alia, on the reactivity ofthe polymerisable mesogenic material, the thickness of the coated layer,the type of polymerisation initiator and the power of the UV lamp. Thecuring time according to the invention is preferably not longer than 10minutes, particularly preferably not longer than 5 minutes and veryparticularly preferably shorter than 2 minutes. For mass productionshort curing times of 3 minutes or less, very preferably of 1 minute orless, in particular of 30 seconds or less, are preferred.

[0066] The inventive polymerisable liquid crystalline mixtures canadditionally comprise one or more other suitable components such as, forexample, catalysts, sensitizers, stabilizers, inhibitors, co-reactingmonomers, surface-active compounds, lubricating agents, wetting agents,dispersing agents, hydrophobing agents, adhesive agents, flow improvers,defoaming agents, deaerators, diluents, reactive diluents, auxiliaries,colourants, dyes or pigments.

[0067] In particular the addition of stabilizers is preferred in orderto prevent undesired spontaneous polymerisation of the polymerisablematerial for example during storage. As stabilizers in principal allcompounds can be used that are known to the skilled in the art for thispurpose. These compounds are commercially available in a broad variety.Typical examples for stabilizers are 4-ethoxyphenol or butylatedhydroxytoluene (BHT).

[0068] Other additives, like e.g. chain transfer agents, can also beadded to the polymerisable LC material in order to modify the physicalproperties of the resulting polymer film. When adding a chain transferagent, such as monofunctional thiol compounds like e.g. dodecane thiolor multifunctional thiol compounds like e.g. trimethylpropanetri(3-mercaptopropionate), to the polymerisable material, the length ofthe free polymer chains and/or the length of the polymer chains betweentwo crosslinks in the inventive polymer film can be controlled. When theamount of the chain transfer agent is increased, the polymer chainlength in the obtained polymer film is decreasing.

[0069] It is also possible, in order to increase crosslinking of thepolymers, to add up to 20% of a non mesogenic compound with two or morepolymerisable functional groups to the polymerisable LC materialalternatively or in addition to the di- or multifunctional polymerisablemesogenic compounds to increase crosslinking of the polymer. Typicalexamples for difunctional non mesogenic monomers are alkyldiacrylates oralkyldimethacrylates with alkyl groups of 1 to 20 C atoms. Typicalexamples for non mesogenic monomers with more than two polymerisablegroups are trimethylpropanetrimethacrylate orpentaerythritoltetraacrylate.

[0070] In another preferred embodiment the mixture of polymerisablematerial comprises up to 70%, preferably 3 to 50% of a non mesogeniccompound with one polymerisable functional group. Typical examples formonofunctional non mesogenic monomers are alkylacrylates oralkylmethacrylates.

[0071] It is also possible to add, for example, a quantity of up to 20%by weight of a non polymerisable liquid-crystalline compound to adaptthe optical properties of the resulting polymer film.

[0072] The polymerisation is preferably carried out in the liquidcrystal phase of the polymerisable LC material. Therefore, preferablypolymerisable mesogenic compounds or mixtures with low melting pointsand broad liquid crystal phase ranges are used. The use of suchmaterials allows to reduce the polymerisation temperature, which makesthe polymerisation process easier and is a considerable advantageespecially for mass production. The selection of suitable polymerisationtemperatures depends mainly on the clearing point of the polymerisablematerial and inter alia on the softening point of the substrate.Preferably the polymerisation temperature is at least 30 degrees belowthe clearing temperature of the polymerisable mesogenic mixture.Polymerisation temperatures below 120° C. are preferred. Especiallypreferred are temperatures below 90° C., in particular temperatures of60° C. or less.

[0073] For the substrate or surface onto which the birefringent markingis applied, any type of material can be used, like for example glass orquarz substrates, plastic films or sheets or metal surfaces.

[0074] The birefringent markings according to the present invention areespecially suitable for use in hot stamping foils and holographic foilsfor the preparation of security markings and security threads. Thepreparation of holographic layers is described e.g. in U.S. Pat. No.4,588,664, the entire disclosure of which is incorporated into thisapplication by way of reference. Hot stamping foils comprising liquidcrystal material are described in the patent application GB 2 357 061.

[0075] Furthermore, for decorative applications the birefringent markingaccording to the invention can be directly applied to decoratingobjects, car bodies or any object with a reflective surface by themethod described above. Thus, metal objects like e.g. car bodies,household objects, lamps or other objects can be coated or printeddirectly with droplets of an LC mixture according to the inventivemethod.

[0076] The inventive birefringent marking can be used for directapplication, or as holograms or hot stamping foils for decorative orsecurity applications, to authenticate and prevent counterfeiting ofdocuments of value, for identification of hidden images, informations orpatterns. They can be applied to consumer products or household objects,car bodies, foils, packing materials, clothes or woven fabric,incorporated into plastic, or applied as security markings or threads ondocuments of value like banknotes, credit cards or ID cards, national IDdocuments, licenses or any propduct with money value, like stamps,tickets, shares, cheques etc.

[0077] Without further elaboration, it is believed that one skilled inthe art can, using the preceding description, utilize the presentinvention to ist fullest extent. The following examples are, therefore,to be construed as merely illustrative and not limitative of theremainder of the disclosure in any way whatsoever.

[0078] In the foregoing and in the following examples, unless otherwiseindicated, all temperatures are set forth uncorrected in degrees Celsiusand all parts and percentages are by weight.

EXAMPLE 1

[0079] The following polymerisable mixture is prepared Compound (A)3.02% compound (B) 7.60% compound (C) 5.70% compound (D) 7.04% Irgacure907 1.60% Fluorad FC 171 0.15% Toluene 55.93%  Propan-2-ol 18.97% 

(A)

(B)

(C)

(D)

[0080] Compounds (A), (B) and (D) can be prepared according to or inanalogy to the methods described in D. J. Broer et al., Makromol. Chem.190, 3201-3215 (1989). Compound (C) and its preparation are described inGB 2,280,445. Irgacure 907 is a commercially available photoinitiator(Ciba Geigy). Fluorad FC 171 is a commercially available surfactant(DuPont).

[0081] Droplets of the mixture were placed onto the metallised side of ametallised PET film. The droplets were left to stand for 2 minutes toallow the solvent to evaporate, leaving behind a liquid crystallinecoating. The coating is then cured in an air environment at roomtemperature to cure the liquid crystal mixture. This forms a polymerfilm which has variable film thickness.

[0082] When viewed through a linear polariser, a distinct 4-spokedpattern of colour is seen as shown in FIG. 1. This pattern rotates uponrotation of the polariser.

[0083] In addition a pool of liquid crystalline mixture was left toevaporate. When all solvent had been removed a rubber stamp was pressedinto the mixture and then stamped onto a similar piece of metallised PETas used before. The film was then exposed to UV radiation to cure thestamped film. When viewed through a linear polariser coloured regionscould be seen in the stamped area. The thickness of the stamped film wasapproximately 2 μm. Thinner areas showed no colour.

[0084] The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

[0085] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. Birefringent marking comprising at least one layer of liquid crystalmaterial having regions of different thickness.
 2. Birefringent markingaccording to claim 1, wherein the liquid crystal material is avitrified, polymerised or crosslinked liquid crystal material. 3.Birefringent marking according to claim 1 or 2, wherein the liquidcrystal material is a nematic or smectic liquid crystal material. 4.Birefringent marking according to claim 1 or 2, wherein the liquidcrystal material is a cholesteric liquid crystal material. 5.Birefringent marking according to any of claims 1 to 4, furthercomprising a reflective layer.
 6. Birefringent marking according toclaim 5, wherein the reflective layer is a metallic or metallized layer,hot stamping foil, holographic image, pearlescent or interference layeror pearlescent or interference pigment.
 7. Birefringent markingaccording to claim 4, further comprising an absorptive layer. 8.Birefringent marking according to any of claims 1 to 7, furthercomprising at least one optical retardation layer.
 9. Method ofpreparing a birefringent marking according to any of claims 1 to 8 byapplying droplets of a solution of a liquid crystal material to asubstrate and allowing the solvent to evaporate.
 10. Method according toclaim 9, wherein the liquid crystal material is a polymerisable orcrosslinkable material that is polymerised or crosslinked during orafter evaporation of the solvent.
 11. Method of applying a hidden imageto a surface or substrate, said image being invisible when viewed underunpolarised light and visible when viewed under polarised light, byproviding a birefringent marking comprising a liquid crystal materialhaving regions of different thickness to the surface.
 12. Use of abirefringent marking according to any of claims 1 to 8 in decorative orsecurity applications.
 13. Security, authentification or identificationmarking or device comprising a birefringent marking according to any ofclaims 1 to
 8. 14. Document of value comprising a marking or deviceaccording to claim 13.